2010年4月18日 星期日

Disaster

In 1991, Mount Pinatubo on the island of Luzon, in the Philippines, erupted just 90km (55 miles) north-west of the capital, Manila.

Over the course of several eruptions, the volcano ejected a massive 10 cubic km of material, making it the second biggest eruption of the 20th Century.

While volcanic dust is in the upper atmosphere, it can have a profound short-term effect on the global climate, because it blocks out a portion of the sunlight able to reach the ground.

The Mount Pinatubo eruption caused the average global temperature to drop by 0.4-0.5C.

Previous eruptions have been much more deadly. The eruption of the volcano on Krakatoa in Indonesia in 1883 is one of the best known eruptions in relatively recent times, because it occurred after the invention of the telegraph.

Animated guide: Volcanoes
It killed thousands, pulverised two-thirds of the island and drastically altered the ocean floor. But Krakatoa was a mere baby compared with the eruption of Tambora, also in Indonesia, some 68 years earlier.

This was the most powerful eruption in recorded history. Rivers of hot ash rolled down the 4,000m (13,000ft) -high volcano, killing around 10,000 people on the island.

It is thought to have ejected 50 cubic km or more of material and pumped vast amounts of sulphur dioxide into the atmosphere.

The cloud from Tambora caused an unusual chill, lowering global temperatures by an estimated 0.4-0.7C.

In parts of Europe and in North America, 1816 became known as "the year without a summer". Frosts killed off crops in New England and Canada; Europe was also hit badly.

Cultural effects
The event has even left its legacy in technology and the arts.

The volcanic cloud from Tambora is thought to have been responsible for the unusual, yellow-tinged sunsets painted by JMW Turner. And the unseasonable weather trapped Mary Shelley and her husband Percy in Lord Byron's house on Lake Geneva.

To divert his guests, Lord Byron suggested a writing competition, the most notable result of which was Mary Shelley's Frankenstein.

In Europe, the soaring price of oats - which were fed to horses - may have prompted German inventor Karl Drais to invent a horseless form of transport: the velocipede, which was a direct ancestor of the bicycle.

If one ventures even further back in time, to around 70,000 years ago, a massive volcanic eruption may even have threatened the existence of humankind.

The "super-eruption" of Mount Toba on the Indonesian island of Sumatra is thought by some to have caused a six-year long volcanic winter followed by a 1,000-year-long freeze. Super-eruptions are so-called if they eject more than 1,000 cu km (240 cu miles) of material - far more than any event in recorded history.

Toba could have caused a mass die-off of plant life and a famine for animal species. There is evidence of a major "bottleneck" in the DNA of human populations, which means that genetic variation was drastically reduced.

Human bottleneck
Some researchers have calculated that the human population dropped to between 10,000 and 5,000 individuals, pushing Homo sapiens to the brink of extinction.

Some scientists have challenged the link between Toba and the genetic bottleneck. But others think the near extinction of human life may have pushed people to new levels of ingenuity in order to survive; evidence of advanced tool technology and some very early examples of art have been dated to around the time of the Toba super-eruption.

Further back in history of course, massive volcanic episodes have been linked to mass extinctions of life on Earth.

While mass volcanism at the Deccan traps in India was once considered as a cause of the demise of the dinosaurs 65 million years ago, a comet or asteroid is now considered the most likely candidate for wiping out the beasts.

However, recent research suggests that a large bout of volcanic activity some 200 million years ago may have contributed to the dinosaurs' dominance, by wiping out their competitors.

The most severe extinction event in the geological record - the Permian-Triassic mass extinction, 250 million years ago - may also have been caused by the eruption of the Siberian traps.

Super-eruptions, like the one at Toba, Indonesia, still pose a threat. Yellowstone, in the US state of Wyoming, is often referred to as a super-volcano and last erupted 650,000 years ago. Seismologists continue to monitor the changes at Yellowstone, but any future eruption could be hard to predict.